[en] In this work, an electrochemical sensor coupled with an effective flow-injection amperometry (FIA) system is developed, targeting the determination of gallic acid (GA) in a mild neutral condition, in contrast to the existing electrochemical methods. The sensor is based on a thin electroactive poly(melamine) film immobilized on a pre-anodized screen-printed carbon electrode (SPCE*/PME). The characteristics of the sensing surface are well-characterized by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and surface water contact angle experiments. The proposed assay exhibits a wide linear response to GA in both pH 3 and pH 7.0 phosphate buffer solutions (PBS) under the optimized flow-injection amperometry. The detection limit (S/N = 3) is 0.076 μM and 0.21 μM in the pH 3 and pH 7 solutions, respectively. A relative standard deviation (RSD) of 3.9% is obtained for 57 successive measurements of 50 μM GA in pH 7 solutions. Interference studies indicate that some inorganic salts, catechol, caffeine and ascorbic acid do not interfere with the GA assay. The interference effects from some orthodiphenolic compounds are also investigated. The proposed method and a conventional Folin–Ciocalteu method are applied to detect GA in green tea samples using the standard addition method, and satisfactory spiked recoveries are obtained. - Highlights: • A nitrogen-rich conducting polymer was used for electroanalysis of gallic acid. • The sensor exhibits excellent electrochemical activity in both acidic and neutral media. • Good analytical results in terms of low detection limit and wide linear range are obtained. • The flow-injection amperometric assay is highly stable for continuous 57 replicates measurement (RSD = 3.9%). • The assay shows good recovery for green tea samples.

[en] Highlights: • A novel overoxidized poly(3,4-ethylenedioxythiophene) film is obtained. • The film structure favors the adsorption of dopamine. • The sensor is able to detect dopamine in the presence of ascorbic acid (1000X). - Abstract: An overoxidized poly(3,4-ethylenedioxythiophene) film-modified screen-printed carbon electrodes (SPCE/PEDOT_o_x) was prepared and characterized by field emission scanning electron microscopy (FE-SEM), X-ray photoelectron spectroscopy (XPS) and water contact angle techniques. The obtained film is a porous structure with highly abundant oxygen functionality. The SPCE/PEDOT_o_x could adsorb cations strongly and perform catalytic oxidation of biomolecules. The potential-induced adsorption of dopamine was observed for SPCE/PEDOT_o_x. A simple medium-exchange procedure was developed for the selective determination of dopamine by the use of the dopamine-adsorbed electrode. Under optimal differential pulse voltammetry (DPV), the proposed assay can be employed in the determination of submicromolar concentration of dopamine without the coexisting interferences of ascorbic acid (1000-fold) and uric acid (10-fold)

[en] Highlights: • Electrochemically induced formation of polymer films from CAF and ASP is reported. • The electropolymerization efficiency depends on the activity of the carbon surfaces. • Highly sensitive ASP assay was developed. • The ASP assay has good selectivity against common additives and three artificial sweeteners. • Good recoveries were achieved for Coca-Cola and Coke Zero samples. -- Abstract: Cyclic voltammetry (CV) in a pH 7.0 phosphate buffer solution containing aspartame (ASP) and caffeic acid (CAF) was studied. In sharp contrast to untreated electrodes, two distinct redox peaks were observed at preanodized screen-printed carbon electrodes (SPCE*) due to the formation of a homopolymer poly(CAF) and a copolymer poly(CAF-ASP). The characteristics of the polymer film-modified electrodes were well-characterized by atomic force microscopy (AFM), scanning electron microscopy (SEM), photoelectron spectroscopy (XPS) and the water contact angle technique. Electrochemically induced polymerization mechanisms were discussed. Current responses from poly(CAF-ASP) were employed for ASP determination, and a linear dynamic range from 0.05 μM to 10 μM was obtained using differential pulse voltammetry (DPV). The limit of detection (LOD, S/N = 3) was 0.0076 μM, the limit of quantification (LOQ, S/N = 10) was 0.0256 μM, and sensitivity was 12.067 μA μM⁻¹. The relative standard deviation (RSD) was less than 5%, indicating good precision. Common additives in soft drinks do not interfere with the ASP analysis. The proposed assay was applied for the determination of ASP in two soft drinks using the standard addition method, and recoveries in the range of 103.0–106.7% and 97.3–106.4% were obtained for Coca-Cola and Coke Zero, respectively. A reference high-performance liquid chromatography (HPLC) was applied to validate the proposed method, and good agreement was obtained.

[en] In this study, a hybrid nanocomposite consisting of a conducting polymer and gold nanoparticles (AuNPs) is fabricated onto a screen-printed carbon electrode (SPCE). A thin layer of poly(3,4-ethylenedioxythiophene) (PEDOT) is coated electrochemically on a bare SPCE; then, the nano-sized AuNPs are embedded by electrochemical deposition. The resultant SPCE/PEDOT/AuNPs-modified electrode is characterized by electrochemical methods, field emission scanning electron microscopy (FE-SEM) and X-ray photoelectron spectroscopy (XPS). The SPCE/PEDOT/AuNPs-modified electrode possesses great catalytic activity for the oxidation of cysteine in various pH buffer solutions (pH 2.0-8.0). The selectivity of the method is demonstrated by the separation of the oxidation peaks at up to 240 mV for cysteine and glutathione in pH 6.0 buffer solutions. The effects of the oxidizable interferences are also investigated. Flow-injection amperometry is performed for 0.5-200 μM of cysteine in pH 4.0 buffer solutions, and a linear calibration plot with a slope of 0.115 μA/μM is obtained. The detection limit (S/N = 3) is 0.05 μM. Additionally, the proposed methods obtain satisfactory results in the detection of cysteine-containing medicine samples.